The present invention is directed to a method and apparatus for feeding sheets in a photocopying machine. More specifically, the present invention is directed to a sheet feeder system that uses a microprocessor controller to monitor the operation of a sheet feeder apparatus, checking for abnormal operating conditions such as misfeeds and multifeeds. When such conditions occur, the microprocessor controller causes adjustment of the retard nip normal force on the sheet to be fed, taking into consideration characteristics of the sheet, to correct the condition without operator intervention.
In a typical photocopying machine, sheets are contained in a sheet tray and are fed one-by-one into the photocopying apparatus. An individual sheet is taken from the top of a stack by a feed roll that engages the top surface of the sheet and rotates to strip the sheet away from the stack and into a sheet path. Such sheet feeder systems often include a sensor capable of detecting the presence of a sheet, positioned in the sheet path downstream from the feed roll and coupled to a microprocessor controller for the photocopying machine. Such sensors may be used to detect misfeeds in the following manner. The microprocessor controller, after signaling the feed roll to feed a sheet, measures the length of time between this signal and a return signal issued by the misfeed sensor upon detecting a sheet. If the time measured exceeds a predetermined maximum, the microprocessor controller recognizes a misfeed condition. In a typical photocopying machine, the microprocessor controller then signals the operator that a sheet has been misled and discontinues operation until the operator manually clears the misled sheet.
Misfeeds can occur in a photocopying machine of this type for various reasons including edge welds, static friction peaks and friction degradation in the feed components. A related problem associated with the operation of a typical sheet feeder system is multifeed, where multiple sheets are simultaneously fed through the sheet path instead of a single sheet. Thus, a primary goal in designing sheet feeder systems is to improve the system's ability to handle a variety of sheet types without misfeeds or multifeeds.
U.S. Pat. No. 4,561,644 to Clausing, the disclosure of which is incorporated in its entirety herein by reference, describes a sheet feeding and separating apparatus wherein the normal force applied to a stack of sheets by a feed mechanism is adjusted to reduce misfeeds and multifeeds. The apparatus includes a sensor similar to the misfeed sensor described above. If the sensor fails to detect a sheet within a predetermined time, the normal force exerted against the sheet stack is increased to correct the misfeed condition. The sheet feeding and separating apparatus is mounted on a frame that pivots about an axis. Upon detection of the sheet by the sensor, a solenoid is actuated to cause the frame to pivot, thereby decreasing the normal force against the stack to avoid multifeeds.
While the prior art discloses sheet feeder systems that attempt to reduce the amount of operator intervention required to correct abnormal feed conditions, such systems generally function in a trial-and-error manner in that they are not sensitive to characteristics of the sheets being fed or to recurring problems. There is a need for an "intelligent" photocopying machine capable of establishing an initial setup based on the type of sheet to be fed, actively monitoring operation of the sheet feeder system, making incremental adjustments in response to abnormal feed conditions, and modifying the initial setup to correct recurring problems. This and other needs are satisfied by the sheet feeder apparatus and method of the present invention.